Balun advice, 1:1 common mode choke to remove CM noise from ladderline doublet

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N1VAU, Jan 7, 2020.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    The point of the choke balun (aka current mode balun) is it forces a balanced current condition on the balanced feed line. IOW, if you do indeed have common mode issues and not other reasons to explain the higher noise on your Doublet and you do tame that noise by adding more choking at the junction between the tuner and the balanced feed line. Then not only will noise ingress be reduced but you'll equally reduce any signal radiation from the feed line when you transmit. Noise ingress due to feed line imbalance and feed line transmission due to imbalance are flip sides of the same coin.

    Basically common mode currents on balanced feed line equal the residual when you take a vector sum of the currents on each branch of the line. If the currents are equal in magnitude and opposite in phase on the two balanced feed line conductors then their vector sum would be zero and there would be no common mode current. That's the completely balanced condition. When you have imbalance the two don't completely cancel and that residual when you sum the two currents IS the common mode current.

    If you introduce enough common mode impedance to successfully choke that current (or reduce it to negligible levels) you force the system back into balance. If the residual is forced close to zero then the two conductors have to be forced back into a balanced (or very nearly balanced with a negligible remainder) state to satisfy basic circuit principles like Kirchoff's current law or stated even more basically to satisfy conservation of charge.

    Bottom line, sufficient common mode rejection via choking improves both the receive and transmit performance of the feed line from the perspective of common mode issues.
    WB2UAQ and N1VAU like this.
  2. AA5MT

    AA5MT Ham Member QRZ Page

    When all else fails, try something different.

    I have been quite successful using a noise canceler. I use the MFJ variety. I have a local AM broadcast station which is S9+40 here. With the noise canceler, I can completely null out the station. I had to modify the canceler to go that low in frequency. It has a high pass filter which cuts off about 1.6 MHz. I went online and found out how to remove the filter. It's not for everyone as it's a little touchy. But once you understand it, it's not too bad. So, I get over 100 db null on the interference. THEN, what happens is, the radio agc kicks out, bringing the radio gain up over 100 db from before.! They don't tell you about that part. It's very much the same principal as noise cancelling headphones.

    It's not for everyone, especially if you have two noise sources. It's more a brute force method. Some friends of mine were camped out in Tampa hf under the 440 volt lines. You could see the insulators sparking from salt buildup on the insulators. I let them borrow mine. They told me that it did not completely drop the noise since it was from multiple directions. But it did make the difference whether they operated that night, or not.


  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    So this illustrates the development of CM current on balanced line due to imbalance in the antenna wires them selves. Look what you can learn with a simple model and EzNec...

    The first picture is a 100ft doublet (wires #4 plus #5), center-fed by 50ft of 4inch spaced balanced line (wires #1 and #2). A current source (red circle), simulating a perfect link-coupled tuner, is in the center of 4inch long wire #3 at the bottom of the feedline. This is the perfectly balanced case.

    The spacing of the pink lines relative to the green dots shows the relative current magnitude along each wire. Notice that there is no discontinuity where the parallel feed lines joins the doublet, i.e. per Kirchoff's Current Law, the currents on either side of the feedpoint must be equal:

    So I made wire #4 10% longer, and wire #5 10% shorter. Now there is an intrinsic unbalance in the doublet leg currents at the feedpoint, and you can see the CM current (difference between the currents in wires #1 and#2).

    Notice that the currents do match on either side of the ladder line right above the feedpoint (wire #3), but the CM current standing wave develops from there up. The simulated link-coupled source cannot control the CM current further up the feedline....

    Link coupling myth debunked: Imagine that there is a noise source a few feet from the "tuner" illuminating both sides of the ladder line. Do you think you will get cancellation? Hell no!

    So in the third figure (below), I added a Common-Mode choke right below the feedpoint. Think of the CM choke as being an ideal transformer disposed so as to force the currents in wires #1 and #2 to be equal just below the feedpoint. The simulated CM Choke is shown by the red box with X. Notice that the CM current standing wave on the Ladder line is almost gone, and that the currents on either side of the feedpoint are now equal again, regardless of the doublet imbalance.

    A CM choke at the antenna feedpoint is much more effective in blocking CM current (and minimizing noise pickup) than either a link-coupled tuner or a conventional tuner with a CM choke (balun) right at the tuner output....
  4. K7URX

    K7URX Ham Member QRZ Page

    Now you just need a common mode choke that works effectively from 80 to 10 meters.
    AI3V likes this.
  5. K7URX

    K7URX Ham Member QRZ Page

    And who feeds a doublet off-center? I certainly dont.
    AI3V likes this.
  6. AI3V

    AI3V Ham Member QRZ Page

    And a magic balun that can force equal power into a unbalanced load...

    (Or in this case a magic balbal)


  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    How many times to I have to tell you, a ferrite toroid Common Mode Choke with two wires passing through the aperture forces equal currents in the two wires; not equal power! ...and it does that very well, over a decade of frequencies
  8. K7URX

    K7URX Ham Member QRZ Page

    Wa7ark, your own model shows no significant problems when the doublet is center fed with a link coupled tuner.
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    Am I being too subtle for you???

    The entire purpose of post #33 is to first show the ideal, perfectly balanced case, second to purposely perturb it to make it unbalanced (the easiest method seemed to be to displace the feedpoint a bit) , and third to show that adding a CM choke at the feedpoint re-establishes balance regardless if the double is balanced or not!

    Now you have to convince me that your doublet is perfectly balanced; if you believe it is, I have a bridge to sell you.
  10. K7URX

    K7URX Ham Member QRZ Page

    I doubt it is "perfectly" balanced, but close enough for me. Where is this bridge you have?

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